It has been known for more than fifty years that one of the major causes of hydrocephalus is a disturbance or blockage of the natural sites for resorbing CSF, namely the arachnoid villosities.
In normal adults, the brain is maintained at a constant hydrostatic pressure inside the cranial cavity by means of the self-adjusting mechanism of the arachnoid villosities that open and close appropriately to maintain a constant pressure gradient inside the cerebral ventricles. As a result, any disturbance in CSF resorption will simultaneously give rise to an increase in the volume of the cerebral ventricles, and in most cases to an increase in intraventricular pressure.
In newborns whose frontanelles are still open, hydrocephalus is attended by an increase in the volume of the skull caused by the increase in intraventricular pressure.
In adults, because the skull is rigid, hydrocephalus is revealed by disorders in gait control, incontinence, mental disorders, and a gradual deterioration of the cerebral parenchyma.
A known method of treating hydrocephalus consists in diverting CSF from the cerebral ventricles to some other resorption site such as the heart or the peritoneum.
To do this, a “proximal” or “ventricular” catheter is introduced into one of the ventricles of the cranial cavity by trepanning the patient's skull, and this catheter is connected to a “distal” or drainage catheter which is inserted under the scalp and leads either to the heart or to the peritoneum.
The purpose of the arachnoid villosities is to maintain a constant pressure difference between the CSF and venous blood so as to avoid ventricles being drained off while in the orthostatic position. Thus, when a drainage catheter is installed, it is necessary for the proximal catheter under the scalp to include a pressure-regulating valve intended to restore the proper pressure gradient inside the cranial cavity.
Several types of valve have already been proposed, such as those of US patents Nos. U.S. Pat. Nos. 3,288,142 and 3,527,226 in which the pressure-regulating valve includes a liquid-pressure regulating device that consists in a spherical ball co-operating with a conical seat, said ball being held in place by a pre-calibrated spring so as to open at a preset pressure. Unfortunately, that type of valve responds at a single predetermined pressure difference only, which means that a complete series of valves must be provided to a neurosurgical department to make it possible to operate on the variety of pressure ranges that is consistent with the various cases of hydrocephalus that are to be treated. Furthermore, it is to be expected that during evolution of the disease, a previously-implanted valve will be set to open at a pressure that becomes too high or, on the contrary, too low. In either case the valve must be replaced, thus involving another operation.
To solve that problem, a programmable valve as described in European patent No. EP 0 060 369 (U.S. Pat. No. 4,443,214) discloses the possibility of adjusting opening pressure from the outside in “non-invasive” manner so that the valve can be caused to open at a pressure desired by the neurosurgeon, as a function of the physiological condition of the patient.
That programmable valve consists in a valve body of flat cylindrical shape with two opposite projections respectively constituting inlet and outlet ducts for CSF flow. At the inner end of the inlet duct, a frustoconical seat is provided for engaging a corresponding spherical ball which is urged against its seat by a curved spring blade extending along a portion of the inside wall of the valve body. One end of the curved spring blade is fastened to the end of a diametrally-extending rotary magnetic bar (or “rotor”).
The neurosurgeon can make use of an outside magnet acting through the scalp of the patient and the wall of the valve to turn the magnetic bar into a desired position. In other words, by turning the magnetic bar, the length of the active portion of the spring blade which acts on the spherical ball can be varied, thereby enabling the valve opening pressure to be adjusted.
The rotor and the spring blade are immobilized by indexing means which act between the end of the rotor bar and the inside of the valve body, thereby providing a plurality of indexed positions corresponding to different preset opening pressures.
Such immobilization of the rotor is entirely satisfactory in most cases, however some very powerful magnetic fields, such as those used in nuclear magnetic resonance (NMR) procedures, can change the position of the rotor. Therefore, after performing such a procedure, the position of the rotor must be checked on the patient and readjusted if it is found to have changed.
Another drawback of the valve described above is the diametral position of the rotor which extends across the valve body, i.e. across the flow of CSF, thus to some extent impeding the free flow of fluid through the valve from its inlet duct to its outlet duct.
Furthermore, it is known that in hydrocephalus, the protein content of CSF increases and can sometimes reach as much as 2% by weight (2 grams (g) per hundred grams) or even more, as reported in the literature. As a consequence, as soon as a valve presents one or more internal sites where it can retain CSF, the risks of clogging increase.
To solve the problem of the position of the rotor possibly changing under the effect of a powerful magnetic field, a subcutaneous valve has been proposed as described in patent No. FR 2 721 520.
That valve has features similar to those of the valve described in EP 0 060 369, except that the magnetic rotor consists of an H-shaped bar in which the two pairs of side branches on either side of the central axis of rotation act as means for guiding two micromagnets. These two micromagnets have facing faces of the same polarity, and are suitable for sliding between the branches of said rotor along the longitudinal axis of the bar so as to actuate locking pins which are suitable for co-operating with a series of cavities provided in the cylindrical side face of the valve chamber.
That valve solves the drawback of a possible unexpected change in the rotor position, i.e. in spring position, in the event of a patient being subjected to a strong unidirectional magnetic field, since when one of the two magnets is attracted towards the center of the valve, the other magnet is repelled a little harder into its cavity. As a consequence, both micromagnets cannot be removed simultaneously from their respective cavities.
However, because that device has two sliding parts inside the valve, the number of potential fluid-retaining or “dead” sites is further increased, particularly since the mechanism needs to be extremely miniaturized in order to be integrated in a valve chamber having inside dimensions of centimeter (cm) order. Such a valve therefore risks clogging even more quickly than the valve of document EP 0 060 369.